U.S. patent application number 10/799641 was filed with the patent office on 2004-09-23 for optical pickup apparatus.
This patent application is currently assigned to KONICA MINOLTA HOLDINGS, INC.. Invention is credited to Atarashi, Yuichi, Ikenaka, Kiyono.
Application Number | 20040184386 10/799641 |
Document ID | / |
Family ID | 32984748 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040184386 |
Kind Code |
A1 |
Atarashi, Yuichi ; et
al. |
September 23, 2004 |
Optical pickup apparatus
Abstract
An optical pickup apparatus includes first, second and third
light sources; a light converging optical system including an
objective optical element, converging a light flux emitted from the
first to third light sources respectively onto first to third
information recording surfaces, wherein the light converging
optical system introduces the light flux emitted from the first
light source as an infinite parallel light flux to be incident on
the objective optical element; and a chromatic aberration
correcting element suppressing a variation of a chromatic
aberration based on a wavelength variation in a light flux emitted
from the first light source. The light converging optical system
includes a spherical aberration correcting structure to correct a
spherical aberration caused by at least one of a difference in
thickness among the first to third protective layers and a
difference in wavelength among light fluxes from the first to third
light sources.
Inventors: |
Atarashi, Yuichi; (Tokyo,
JP) ; Ikenaka, Kiyono; (Tokyo, JP) |
Correspondence
Address: |
Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
1300 I Street, N.W.
Washington
DC
20005-3315
US
|
Assignee: |
KONICA MINOLTA HOLDINGS,
INC.
|
Family ID: |
32984748 |
Appl. No.: |
10/799641 |
Filed: |
March 15, 2004 |
Current U.S.
Class: |
369/121 ;
G9B/7.128 |
Current CPC
Class: |
G11B 2007/0006 20130101;
G11B 7/1275 20130101; G11B 7/1392 20130101 |
Class at
Publication: |
369/121 |
International
Class: |
G11B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2003 |
JP |
JP2003-074527 |
Claims
What is claimed is:
1. An optical pickup apparatus comprising: first, second and third
light sources emitting light fluxes having wavelengths of
.lambda.1, .lambda.2 (.lambda.1<.lambda.2) and .lambda.3
(.lambda.2<.lambda.3) respectively; a light converging optical
system including an objective optical element, converging a light
flux emitted from the first light source onto a first information
recording surface on a first optical information recording medium
through a first protective layer with a thickness of t1 so as to
conduct recording or reproducing information for the first optical
information recording medium, converging a light flux emitted from
the second light source on a second information recording surface
on a second optical information recording medium through a second
protective layer with a thickness of t2 so as to conduct recording
or reproducing information for the second optical information
recording medium, and converging a light flux emitted from the
third light source on a third information recording surface on a
third optical information recording medium through a third
protective layer with a thickness of t3 (t1<t3 and t2<t3) so
as to conduct recording or reproducing information for the third
optical information recording medium, wherein the light converging
optical system introduces the light flux emitted from the first
light source as an infinite parallel light flux to be incident on
the objective optical element when information is reproduced from
or recorded on the first information recording medium; and wherein
the light converging optical system includes: a spherical
aberration correcting structure to correct a spherical aberration
caused by at least one of a difference in thickness among the first
to third protective layers and a difference in wavelength among
light fluxes from the first to third light sources; and a chromatic
aberration correcting element arranged in an optical path where a
light flux emitted from the first light source passes and
suppressing a variation of a chromatic aberration based on a
wavelength variation in a light flux emitted from the first light
source.
2. The optical pickup apparatus of claim 1, wherein the thickness
of t1 and t2 satisfy a following relationship:
0.9.multidot.t1<t2<1.1.mul- tidot.t1.
3. The optical pickup apparatus of claim 1, comprising a spherical
aberration correcting element having the spherical aberration
correcting structure in a common path where all of the light fluxes
emitted from the first to third light sources pass.
4. The optical pickup of claim 2, comprising a spherical aberration
correcting element having the spherical aberration correcting
structure.
5. The optical pickup apparatus of claim 1, wherein the objective
optical element has the spherical aberration correcting
structure.
6. The optical pickup apparatus of claim 1, wherein the second and
third light sources are attached on the same base board.
7. The optical pickup apparatus of claim 4, wherein the second and
third light sources are attached on the same base board.
8. The optical pickup apparatus of claim 1, wherein the light
converging optical system introduces the light flux emitted from
the third light source as a finite divergent light flux to be
incident on the objective optical element when information is
reproduced from or recorded on the third information recording
medium.
9. The optical pickup apparatus of claim 7, wherein the light
converging optical system introduces the light flux emitted from
the third light source as a finite divergent light flux to be
incident on the objective optical element when information is
reproduced from or recorded on the third information recording
medium.
10. The optical pickup apparatus of claim 1, wherein the light
converging optical system introduces the light flux emitted from
the second light source as a finite divergent light flux to be
incident on the objective optical element when information is
reproduced from or recorded on the second information recording
medium.
11. The optical pickup apparatus of claim 9, wherein the light
converging optical system introduces the light flux emitted from
the second light source as a finite divergent light flux to be
incident on the objective optical element when information is
reproduced from or recorded on the second information recording
medium.
12. The optical pickup apparatus of claim 11 wherein the finite
divergent light flux which is incident into the objective optical
element in case that information is reproduced from or recorded on
the second information recording medium has a smaller divergent
angle than the finite divergent light flux which is incident into
the objective optical element in case information is reproduced
from or recorded on the third information recording medium.
13. The optical pickup apparatus of claim 1, wherein the light
converging optical system comprises a collimator, and light fluxes
emitted from the first to third light sources pass through the
collimator toward the objective optical element.
14. The optical pickup apparatus of claim 1, wherein the first to
third light sources are arranged with the same distance from the
objective optical element.
15. The optical pickup apparatus of claim 1, wherein the chromatic
aberration correcting element is at least one of a beam expander, a
collimator and a coupling lens.
16. The optical pickup apparatus of claim 15, wherein the chromatic
aberration correcting element is a beam expander.
17. The optical pickup apparatus of claim 11, wherein the chromatic
aberration correcting element is at least one of a beam expander, a
collimator and a coupling lens.
18. The optical pickup apparatus of claim 17, wherein the chromatic
aberration correcting element is a beam expander.
19. The optical pickup apparatus of claim 3, wherein at least a
part of the spherical aberration correcting element is movable
along an optical axis.
20. The optical pickup apparatus of claim 3 wherein the spherical
aberration correcting element is at least one of a beam expander, a
collimator and a coupling lens.
21. The optical pickup apparatus of claim 18, wherein the spherical
aberration correcting element is at least one of a beam expander, a
collimator and a coupling lens.
22. The optical pickup apparatus of claim 20, wherein the spherical
aberration correcting element is a beam expander.
23. The optical pickup apparatus of claim 21, wherein the spherical
aberration correcting element is a beam expander.
24. The optical pickup apparatus of claim 3, wherein the spherical
aberration correcting element is a liquid crystal element.
25. The optical pickup apparatus of claim 3, wherein the spherical
aberration correcting element corrects a spherical aberration
caused by temperature variation in the objective optical
element.
26. The optical pickup apparatus of claim 1, wherein the objective
optical element is made of a plastic material.
27. The optical pickup apparatus of claim 23, wherein the objective
optical element is made of a plastic material.
28. The optical pickup apparatus of claim 26, wherein an incidence
plane of a light flux emitted from the light sources in the
objective optical element is a refracting surface.
29. The optical pickup apparatus of claim 27, wherein an incidence
plane of a light flux emitted from the light sources in the
objective optical element is a refracting surface.
30. The optical pickup apparatus of claim 1, wherein the objective
optical element is made of a glass material.
31. The optical pickup apparatus of claim 1, further comprising an
aperture limiting element.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical pickup apparatus
and an optical element used for it, and particularly, to an optical
pickup apparatus by which, by using light fluxes emitted from 3
light sources whose light source wavelength is different, the
recording and/or reproducing of respective information can be
conducted on 3 different optical information recording media.
BACKGROUND
[0002] Recently, a research and development of a high density
optical disk system by which, by using a blue violet semiconductor
laser, the recording and/or reproducing can be conducted, are
rapidly advanced. As an example, in the optical disk by which the
information recording/reproducing is conducted under the
specification of NA 0.85, and the light surface wavelength of 405
nm, (hereinafter, such an optical disk is called a "high density
DVD" in the present patent specification), on the optical disk of
the diameter 12 cm whose dimension is the same as the DVD (NA 0.6,
light source wavelength 650 nm, storage capacity 4.7 GB), the
recording of the information of 20-30 GB can be conducted per 1
surface.
[0003] Hereupon, by only a fact that the information can be
adequately recorded/reproduced to such a high density DVD, it can
not be said that a value as the product of the optical pickup
apparatus is enough. At present, when an actuality that the DVD or
CD in which various kinds of information are recorded, is sold, is
taken into account, it is not sufficient by only a fact that the
information can be adequately recorded/reproduced to the high
density DVD, and for example, also to the conventional DVD or CD
which possessed by the user, a fact that the information can be
adequately recorded/reproduced in the same manner, leads to
heighten the value of the product as the exchangeable type optical
pickup apparatus. From such a background, a light converging
optical system used for the exchangeable type optical pickup
apparatus is, also to any one of the high density DVD, conventional
type DVD, or CD, it is desired that the information can be
adequately recorded/reproduced. As an example of such a
exchangeable type optical pickup apparatus, for example, it is
written in the following patent Document 1.
[0004] Patent Document 1
[0005] Tokkai No. 2001-43559
[0006] Hereupon, for such a high density DVD, because the usable
light source is limited, the wavelength used for it is almost
determined, however, the specifications such as the protective
substrate thickness, storage capacity, and NA, are not yet
standardized. For example, for the high density DVD, when it is
considered that the recording density is made to be largely
increased, it is desired that, in order to, initially, increase NA
of an objective lens, and lighten as much as possible the
aberration deterioration caused by an accuracy error which becomes
severe accompanied to it, the protective substrate (also called
protective layer) thickness is decreased. Inversely, when NA of the
objective lens is made the same standard as the conventional
optical disk such as the DVD, the physical recording density is not
largely increased, however, because the performance required as the
optical system becomes comparatively loose, the necessity that the
protective substrate thickness is made thin, is lowered. As a
concrete specification, for the thickness of the protective
substrate, the thickness of 0.1 mm, which is thinner than that of
the conventional DVD, or the thickness of 0.6 mm which is the same
as the conventional DVD, is proposed.
[0007] Although the light converging optical system of the optical
pickup apparatus is made simple, in order to attain the recording
and/or reproducing of the high density information, when NA of the
objective lens at the time of use of the high density DVD is made
larger (for example, 0.85) than NA when the conventional DVD is
used, for example, for the objective lens, 3 optical function areas
such as a common area of the high density DVD, DVD and CD, a common
area of the high density DVD and DVD, and an exclusive area of the
high density DVD are provided, and when the transmitting light flux
is made to be flared, there is an engineering by which the
aberration characteristic can be made good in some degree. However,
from a fact in which there is a problem that the specification of
the high density DVD is not standardized as described above, and
the light source of the short wavelength used when the information
is recorded and/or reproduced to the high density DVD, is severe in
the allowance to the refractive index change due to the temperature
change of the optical element, or the fluctuation (mode-hop)of the
light source wavelength, to give the optical characteristic
necessary when the information is adequately recorded/reproduced
also for any one of the high density DVD, conventional type DVD,
and CD, to the single objective lens, is, in any case in the
theory, it can be said that there are various difficult problems in
the actuality. Hereupon, the engineering by which 3 optical
function areas are provided in the above-described objective lens,
is only an example, and what optical function area is provided, is
different depending on the standard of the optical disk.
[0008] Because, for also the optical pickup apparatus itself, there
is a request for the size reduction, weight reduction,
particularly, the thickness reduction, for component parts,
particularly for the optical element, the very severe performance
is required. Generally, when the thickness of the apparatus is
reduced, a working distance (a distance between the objective
optical element and the optical disk) can not be secured long. For
this, when the magnification of the light converging optical system
is increased, the working distance can be increased, however,
because there is a possibility that the image height characteristic
is deteriorated thereby, there is a problem that it is not
preferable. Further, when the working distance difference among the
high density DVD, conventional type DVD, and CD is increased, the
burden of the actuator at the time of focusing is increased, and
the power consumption is also increased.
SUMMARY
[0009] In view of these problems, the present invention is
attained, and an aspect of the present invention is to provide an
optical pickup apparatus by which, even while the limit of the
design work of the objective optical element and production
allowance is lighten, for all of, for example, the high density
DVD, conventional DVD and CD, the information can be adequately
recorded and/or reproduced.
[0010] The optical pickup apparatus of the present invention is
characterized in that: it has the first light source of the
wavelength .lambda.1, the second light source of the wavelength
.lambda.2 (.lambda.1<.lambda.2), the third light source of the
wavelength .lambda.3 (.lambda.2<.lambda.3), and a light
converging optical system including an objective optical element,
and an optical pickup apparatus by which, when the light converging
optical system makes light-converge the light flux from the first
light source onto the first information recording surface of the
first optical information recording medium through a first
protective layer whose thickness is t1, the information can be
recorded or reproduced, further, when the light converging optical
system makes light-converge the light flux from the second light
source onto the second information recording surface of the second
optical information recording medium through a second protective
layer whose thickness is t2, the information can be recorded or
reproduced, and further, when the light converging optical system
makes light-converge the light flux from the third light source
onto the third information recording surface of the third optical
information recording medium through a third protective layer whose
thickness is t3 (t1<t3, and t2<t3), the information can be
recorded or reproduced, and when the information is reproduced or
recorded for the first optical information recording medium, the
infinite-parallel light flux is made incident on the objective
optical element, and the optical pickup apparatus has a function by
which the spherical aberration generated by at least one of the
difference of the first--third protective layer thickness of the
first--third optical information recording media and the wavelength
difference of the first--third light sources, is corrected, and has
a chromatic aberration correction element which is arranged in an
optical path through which the light flux projected from the first
light source passes, and by which the variation of the chromatic
aberration according to the wavelength variation of the first light
source is suppressed.
[0011] The optical pickup apparatus of the present invention has,
when the information is recorded and/or reproduced for 3 kinds of
different optical information recording media, the chromatic
aberration correction element which is separately provided from the
objective optical element, and by which the chromatic aberration
correction due to a condition change is conducted. When the
chromatic aberration correction according to the condition change
is made to be conducted by the chromatic aberration correction
element provided separately from the objective optical element, the
limitation of the design work and the production allowance of the
objective optical element can be lightened. Further, the optical
pickup apparatus of the present invention has a function by which
the spherical aberration due to at least one of the wavelength
difference among a plurality of light sources and the difference of
the protective layer thickness in a plurality of optical
information recording media, is corrected. When such a function is
provided, the recording/reproducing can be conducted on the light
sources having the different wavelengths and the optical
information recording media having the protective layer whose
thickness is different. For such a spherical aberration correction
function, it is also possible to make it have the objective optical
element, however, it is preferable that it is made to conduct it by
the spherical aberration correction element separately provided
from that. When, by the chromatic aberration correction element,
the chromatic aberration correction is conducted, and the spherical
aberration correction is conducted by the spherical aberration
correction element, it is not necessary that the diffractive
structure for the aberration correction is provided in the
objective optical element, and for example, the optical surface
formed of only the refractive surface can be formed, and the
limitation of the design work and the production allowance is
soften, and viewing in the total, the lower cost optical pickup
apparatus is provided. Hereupon, in the present invention, it is
not limited that the chromatic aberration correction element
conducts all the chromatic aberration, but, the objective optical
element may also take charge of a part of the chromatic aberration
correction. In the same manner, the spherical aberration correction
element does not conduct all the spherical aberration correction,
but, the objective optical element may also take charge of a part
of the spherical aberration correction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is an outline sectional view of an optical pickup
apparatus according to the embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] In the present invention, it is preferable that the first
protective layer thickness t1 of the first optical information
recording medium and the second protective layer thickness t2 of
the second optical information recording medium satisfy the
following relationship.
0.9.multidot.t1<t2<1.1.multidot.t1
[0014] In the case where such a relationship is satisfied, when
recording/reproducing is conducted on the first and second optical
information recording media, it is not necessary that the spherical
aberration due to the thickness of respective protective layers is
corrected, and the design work becomes easy.
[0015] Further, the chromatic aberration correction element and the
spherical aberration correction element may be integrated one, or
separated one, however, at least, it is preferable that the
chromatic aberration correction element is arranged in the optical
path through which the light flux projected from the first light
source for which the chromatic aberration correction is most
necessary, passes. On the one hand, the spherical aberration
correction element may also be arranged in any one of a common
optical path through which the light fluxes from each of light
sources pass or single optical path.
[0016] Further, in the present invention, when the information is
reproduced and/or recorded for the first optical information
recording medium, because it becomes most severe aberration
characteristically, when the infinite-parallel light flux is made
incident on the objective optical element, the influence of the
aberration deterioration at the time of tracking can be suppressed
small.
[0017] In the optical pickup apparatus of the present invention,
when the information is reproduced and/or recorded for the third
optical information recording medium, it is preferable that a
finite divergent light flux is made incident on the objective
optical element. By such a structure, at least a part of the over
spherical aberration generated due to the case where the third
protective layer thickness of the third information recording
medium is thicker than that of other optical information recording
media can be cancelled by the under spherical aberration generated
when the finite divergent light flux is made incident on the
objective optical element.
[0018] In the optical pickup apparatus of the present invention,
when the information is reproduced and/or recorded for the second
optical information recording medium, it is preferable that the
finite divergent light flux is made incident on the objective
optical element. By such a structure, at least a part the over
spherical aberration due to a case where the wavelength of the
second light source is longer than that of the first light source,
can be cancelled.
[0019] Further, in the present invention, it is preferable that,
when the information is reproduced or recorded for the second
information recording medium, a diverging angle of the finite
divergent light flux incident on the objective optical element is
smaller than the diverging angle of the finite divergent light flux
incident on the objective optical element when the information is
reproduced or recorded for the third information recording
medium.
[0020] In the optical pickup apparatus of the present invention, it
is preferable in a point in which the number of parts can be
reduced, that the light converging optical system includes a
collimator, and the light fluxes projected from the first light
source, the second light source and the third light source pass
through the same collimator and go forward to the objective optical
element.
[0021] In the optical pickup apparatus of the present invention, it
is preferable in a point in which the reduction to the low cost and
the space saving can be intended, that the second light source and
the third light source are attached to the same substrate.
[0022] In the optical pickup apparatus of the present invention, it
is preferable in a point in which the reduction to the low cost and
the space saving can be more intended, that the first light source,
the second light source and the third light source are arranged in
an equal direction from the objective optical element.
[0023] In the optical pickup apparatus of the present invention, it
is preferable that the chromatic aberration correction element is
at least one of a beam expander, collimator and coupling lens, and
it is more preferable that it is the beam expander. As more
specific structure, the structure such as the diffractive
structure, phase structure, and multi-level can be given to at
least one optical surface of the beam expander, collimator, and
coupling lens.
[0024] In the optical pickup apparatus of the present invention, it
is preferable that at least a part of the spherical aberration
correction element is movable in the optical axis direction. By
such a structure, corresponding to the condition such as the light
source wavelength in the recording and/or reproducing of the
information, magnification, substrate thickness, and temperature,
by moving a part of the spherical aberration correction element,
the spherical aberration correction can be freely conducted.
[0025] In the optical pickup apparatus of the present invention, it
is preferable that the spherical aberration correction element is
at least one of the beam expander, collimator, coupling lens, and
objective optical element, and it is more preferable that it is the
beam expander. As the more specific structure, the structure such
as the diffractive structure, phase structure, and multi-level can
be given to at least one optical surface of the beam expander,
collimator, coupling lens and objective optical element.
[0026] In the optical pickup apparatus of the present invention, it
is also preferable that the spherical aberration correction element
is a liquid crystal element. Corresponding to the condition such as
the light source wavelength in the recording and/or reproducing of
the information, magnification, substrate thickness, and
temperature, by moving the liquid crystal element, the spherical
aberration correction can be freely conducted. As an example of a
liquid crystal element, there is an element having the structure
laminated in the order of an insulation substrate (for example, a
glass substrate), electrode, liquid crystal molecule layer,
electrode, insulation substrate (for example, a glass substrate),
and in such a liquid crystal element, at least one of electrodes is
divided into the ring-shaped zone pattern around the optical axis.
By using a spherical aberration change signal of a light converging
spot on the information recording surface generated according to
the output signal of a light detection unit, when a predetermined
voltage is impressed in the electrode onto the electrode divided
into the ring-shaped pattern in this manner, an arrangement pattern
of a liquid crystal molecule layer is changed ring-shaped
zone-likely, as a result, a ring-shaped zone-like refractive index
distribution around the optical axis can be given to the liquid
crystal element. Because the spherical aberration is added to the
wave front of the light flux transmitted through the liquid crystal
element having such a ring-shaped zone-like refractive index
distribution, thereby, the spherical aberration change generated by
the wavelength change of the light source (semiconductor laser)
accompanied to the temperature change can be corrected.
[0027] In the optical pickup apparatus of the present invention, it
is preferable that the spherical aberration correction element
corrects the spherical aberration generated corresponding to the
temperature change of the objective optical element.
[0028] In the optical pickup apparatus of the present invention, it
is preferable that the objective optical element is formed of
plastics as a raw material.
[0029] In the optical pickup apparatus of the present invention, it
is preferable that the objective optical element has the glass as a
raw material.
[0030] In the optical pickup apparatus of the present invention, it
is preferable that the apparatus has an aperture limit element by
which the light flux can be stopped down corresponding to the
numerical aperture necessary for the optical information recording
medium. As the aperture limit element, there is a stop in which the
stop diameter is changed corresponding to the wavelength, or an
optical element in which a dichroic-coat is given onto the optical
surface. When the aperture limit element can be used in combination
with the spherical aberration correction element or chromatic
aberration correction element, the number of parts can be
reduced.
[0031] In the present specification, an objective optical element
indicates, in a narrow meaning, an optical element (for example,
lens) having the light converging action, which is arranged to be
faced to a position of the most optical information recording
medium side in the situation in which the optical information
recording medium is loaded into the optical pickup apparatus, and
indicates, in a wide meaning, an optical element which is movable
at least in the optical direction by an actuator together with the
optical element. Accordingly, in the present specification, a
numerical aperture NA on the optical information recording medium
side (image side) of the optical element indicates the numerical
aperture NA of the surface positioned on the most optical
information recording medium side of the optical element. Further,
a necessary numerical aperture NA in the present specification is
defined to indicate a numerical aperture regulated by the
regulation of respective optical information recording media, or a
numerical aperture of the objective optical element of the
diffraction limit performance by which, to respective optical
information recording media, corresponding to the wavelength of the
used light source, a spot diameter necessary for the recording or
reproducing of the information can be obtained.
[0032] A diffractive structure used in the present specification
means a form in which, on the surface of the optical element, for
example, on the surface of the lens, a relief is provided, and to
which an action by which the light flux is converged or diverged by
the diffraction, is given, and when there are an area in which the
diffraction is generated, and an area in which the diffraction is
not generated, it means the area in which the diffraction is
generated. As a shape of the relief, it is well known that, for
example, on the surface of the optical element, it is formed as an
almost concentric circular ring-shaped zone around the optical
axis, and when its section is viewed in a plane including the
optical axis, each ring-shaped zone has a saw-toothed shape, and it
includes such a shape.
[0033] In the present specification, as an optical information
recording medium, there is particularly no limitation so far as it
satisfies the structure of the present invention, however, for
example, as the first optical information recording medium, a high
density DVD system optical disk, as the second optical information
recording medium, each kind of DVD system optical disk such as a
DVD-ROM used for an exclusive use of reproducing, a DVD-Video and
others, a DVD-RAM combinedly used for both of
reproducing/recording, a DVD-R, a DVD-RW, is used. Further, as the
third optical information recording medium, for example, an optical
disk of a CD system such as a CD-R, and CD-RW is used.
PREFERRED EMBODIMENT OF THE INVENTION
[0034] Referring to drawings, the present invention will be
described in more detail below. FIG. 1 is an outline sectional view
of an optical pickup apparatus by which, for all of the high
density DVD (also called the first optical disk), conventional DVD
(also called the second optical disk) and CD (also called the third
optical disk), the information can be recorded/reproduced,
according to the present embodiment.
[0035] In FIG. 1, the beam shape of the light flux projected from
the first semiconductor laser 101 (the wavelength .lambda.1=380
nm-450 nm) as the first light source, is corrected by a beam shaper
102, the light flux passes through the first beam splitter 103, and
after it is formed into parallel light flux by a collimator 104,
passes through the second beam splitter 105, and is incident on a
beam expander having optical elements 106 and 107. Beam expander
(106, 107) in which at least one of the optical elements
(preferably, the optical element 106) is movable in the optical
axis direction, changes (herein, enlarges) the light flux diameter
of the parallel light flux, and has a function to correct the
spherical aberration. Further, the diffractive structure
(diffractive ring-shaped zone) is formed on the optical surface of
the other optical element 107 of the beam expander, thereby, the
chromatic aberration correction is conducted on the light flux
projected from the first semiconductor laser 101. The diffractive
structure for the chromatic aberration correction may be provided
on not only the optical element 107, but also on the other optical
element (collimator 104).
[0036] As described above, when the beam expander (106, 107) is
provided, the chromatic aberration correction and spherical
aberration correction can be conducted, and further, when the high
density DVD is a type which has the information recording surface
on 2 layers, by moving the optical element 106 in the optical axis
direction, the selection of the information recording surface can
also be conducted. The beam expander (106, 107) is arranged in the
common optical path through which light fluxes from the second
semiconductor laser 201, and the third semiconductor laser 301
pass.
[0037] In FIG. 1, the light flux passes through the beam expander
(106, 107) passes through a stop 108, and by an objective lens 109
which is an objective optical element formed only of the refractive
surface, it is light converged on the first information recording
surface through the first protective layer (thickness t1=0.5-0.7
mm, preferably, 0.6 mm) of the first optical disk 110, and the
light converging spot is formed here. Hereupon, the objective lens
109 may be formed of a glass as a raw material, however, because
the aberration deterioration generated by the environmental change
can be arbitrarily corrected by the beam expander (106, 107), and
because the limitation of the required optical characteristic is
lightened, a raw material of lower cost plastics can be used.
[0038] Then, because the light flux which is modulated by an
information pit and reflected on the first information recording
surface, passes again through the objective lens 109, stop 108, and
beam expander (107, 106), and is reflected by the second beam
splitter 105, the astigmatism is given by a cylindrical lens 111,
and the light flux passes through a sensor lens 112, and is
incident on the light receiving surface of a light detector 113, by
using its output signal, a reading signal of the information
recorded on the first optical disk 110 can be obtained.
[0039] Further, by detecting the shape change of the spot and the
light amount change by the position change on the light detector
113, the focusing detection or track detection is conducted. Based
on this detection, the second dimensional actuator 120 integrally
moves the objective lens 109 in such a manner that the light flux
from the first semiconductor laser 101 is focused on the fist
information recording surface of the first optical disk 110.
[0040] Further, in FIG. 1, the second semiconductor laser 201 and
the third semiconductor laser 301 are attached on the same
substrate, and are formed into a single unit which is so called 2
laser 1 package. The light flux projected from the second
semiconductor laser 201 (the wavelength .lambda.2=600-700 nm) as
the second light source, passes through a 1/4 wavelength plate 202,
passes through the third beam splitter 203, reflected on the first
beam splitter 103, and becomes the parallel light flux while the
light flux diameter is stopped down by the collimator 104, passes
through the second beam splitter 105, and is incident on the beam
expander (106, 107), and converted herein into the finite divergent
light flux having a gentle diverging angle. As described above, the
beam expander (106, 107) can conduct the spherical aberration
correction. Hereupon, to the collimator 104 as an aperture limit
element, a dichroic coat is given, and when a pass-through area of
the light flux is limited corresponding to the wavelength, for
example, for the light flux from the first semiconductor laser 101,
the numerical aperture of the objective lens 109 NA=0.65 is
realized, and for the light flux from the second semiconductor
laser 201, the numerical aperture of the objective lens 109 NA=0.65
is realized, and for the light flux from the third semiconductor
laser 301, the numerical aperture of the objective lens 109 NA=0.45
is realized. Hereupon, a combination of the numerical apertures is
not limited to this.
[0041] In FIG. 1, the light flux passed through the beam expander
(106, 107), passes through a stop 108 under the finite diverging
condition having the gentle diverging angle, and is light converged
on its second information recording surface through the second
protective layer of the second optical disk 110' (thickness
t2=0.5-0.7 mm, preferably, 0.6 mm) by the objective lens 109 formed
of only the refractive surface, and the light converging spot is
formed here.
[0042] Then, because the light flux modulated by the information
pit and reflected on the second information recording surface
passes through again the objective lens 109, stop 108, beam
expander (107, 106), second beam splitter 105, and collimator 104,
reflected by the beam splitter 103, and successively, reflected by
the third splitter 203, after that, the astigmatism is given by the
cylindrical lens 204, and the light flux passes through the sensor
lens 205, and is incident on the light receiving surface of the
light detector 206, by using its output signal, the reading signal
of the information recorded in the second optical disk 110' is
obtained.
[0043] Further, by detecting the shape change of the spot on the
light detector 113, and the light amount change by the position
change, the focusing detection or track detection is conducted.
Based on this detection, the second dimensional actuator 120
integrally moves the objective lens 109 in such a manner that the
light flux from the third semiconductor laser 301 is focused on the
second information recording surface of the second optical disk
110'.
[0044] Further, in FIG. 1, the light flux projected from the third
semiconductor laser 301 (the wavelength .lambda.3=770 nm-830 nm) as
the third light source, passes through a 1/4 wavelength plate 202,
passes through the third beam splitter 203, reflected on the first
beam splitter 103, and becomes the parallel light flux while the
light flux diameter is stopped down by the collimator 104, passes
through the second beam splitter 105, and is incident on the beam
expander (106, 107), and converted herein into the finite divergent
light flux having more intensive (larger) diverging angle than the
case of the light flux of the second semiconductor laser 201. In
the same manner, the beam expander (106, 107) can conduct the
chromatic aberration correction and the spherical aberration
correction.
[0045] In FIG. 1, the light flux passed through the beam expander
(106, 107), passes through the stop 108 under the finite diverging
condition having the intensive diverging angle, and is light
converged on its third information recording surface through the
third protective layer (thickness t3=1.1-1.3 mm, preferably, 1.2
mm) of the third optical disk 110" by the objective lens 109 formed
of only the refractive surface, and the light converging spot is
formed here.
[0046] Then, because the light flux modulated by the information
pit and reflected on the third information recording surface passes
through again the objective lens 109, stop 108, beam expander (107,
106), second beam splitter 105, and collimator 104, reflected by
the beam splitter 103, and successively, reflected by the third
splitter 203, after that, the astigmatism is given by the
cylindrical lens 204, the light flux passes through the sensor lens
205, and is incident on the light receiving surface of the light
detector 206, by using its output signal, the reading signal of the
information recorded in the third optical disk 110" is
obtained.
[0047] Further, by detecting the shape change of the spot and the
light amount change by the position change on the light detector
113, the focusing detection or track detection is conducted. Based
on this detection, the second dimensional actuator 120 integrally
moves the objective lens 109 in such a manner that the light flux
from the second semiconductor laser 201 is focused on the third
information recording surface of the third optical disk 110".
[0048] In the present embodiment described above, when beam
expander (106, 107) is functioned as the chromatic aberration
correction element and spherical aberration correction element, in
which the chromatic aberration correction function and the
spherical aberration correction function are given to the beam
expander, corresponding to the condition such as the light source
wavelength in the recording and/or reproducing of the information
to each of optical disks, magnification, substrate thickness, and
temperature, the chromatic aberration correction and spherical
aberration correction can be freely conducted. Further, thereby,
the design work or production of the objective lens 109 can be
easily conducted. Hereupon, as the spherical aberration correction
element, it is not limited to the beam expander, but the collimator
to which the diffractive structure is provided, and other optical
elements may also be used. Further, in spite of the beam expander,
or in addition to it, the liquid crystal element may also be
provided.
[0049] Referring to the embodiment, the present invention is
described above, however, the present invention is not to be
construed by limiting to the above embodiment, but, it is of course
that the modification and improvement can be adequately
conducted.
[0050] According to the present invention, while the limitation of
the design work and production allowance of the objective optical
element, is lightened, an optical pickup apparatus by which the
information can be adequately recorded and/or reproduced for all
of, for example, the high density DVD, conventional DVD, or CD, is
provided.
* * * * *